A general steam turbine is configured such that a rotor, which is a rotating shaft, is supported rotatably by a casing, turbine blades are provided on an outer peripheral portion of this rotor, turbine vanes are provided on the casing, and multiple stages of these blades and vanes are alternately arranged in a steam passage. Therefore, when steam flows in the steam passage, flow of this steam is regulated by the turbine vanes, and the rotor is able to be driven and rotated via the turbine blades.
In such a steam turbine, the casing is formed of an upper casing and a lower casing. Four curved legged portions are provided around the lower casing, and the lower casing is supported by respective curved legged bases erected on a frame. The upper casing is placed on the lower casing, and connected to the lower casing by bolts. Temperature of the casing of the steam turbine changes upon startup, upon operation, and upon stoppage, and the temperature differs depending on positions thereof. For example, by collection of high temperature air at an upper portion of the casing due to flow of air inside the casing, the upper casing tends to become higher in temperature than the lower casing. The upper casing and the lower casing then undergo different thermal deformations, and thus a clearance between the casing and the turbine may become small.
As means for solving this problem, for example, those described in the following patent literatures are available. In a turbine described in Japanese Patent No. 4410651, a main body portion of the lower casing is covered by a heat insulating material such that it becomes difficult for the main body portion of the lower casing to radiate heat compared to an upper half ground portion. Further, in a steam turbine described in Japanese Patent No. 5159702, height of an upper half casing with respect to a bearing stand is adjusted based on a temperature of the casing. Furthermore, as to bolts for pressing a casing of a steam turbine described in Japanese unexamined Utility Model Application Publication No. 63-063506, an upper half casing and a lower half casing are placed in a bearing box and fastened by hold-down bolts.
According to the above described Japanese Patent No. 4410651 and Japanese Patent No. 5159702, since the heat insulating material or a device for adjusting the height is provided, the structure becomes complicated, and manufacturing cost is also increased. Further, according to Japanese Unexamined Utility Model Application Publication No. 63-063506, when the lower half casing undergoes thermal expansion, the upper half casing is deformed upward, and thus the shaft centers of the casing and the turbine are shifted from each other, and the lower clearance between the casing and the turbine becomes small.